In small digital cameras, the depth of field is relatively wide, due to, among other factors, their short focal length. One example of this group of cameras is mobile stations equipped with a digital imaging capability. The great depth of field makes it difficult to create a blurred background in the image. One example of such an imaging application is portraits. In them, it is only the primary image object that is desired to be shown sharply, the background, i.e. the secondary image objects, being desired to be blurred.
Solutions are known from the prior art, in which a shallow depth of field is implemented by using a large aperture (small F-number) and a long focal length. This arrangement is known, for example, from SLR (Single Lens Reflex) cameras. Another possibility is blurring implemented by post-editing. This is a common functionality, for example, in still-image editors. US patent publication US-2002/0191100 A1 (Casio Computer Co. Ltd.) discloses one background-blurring method performed in a camera device in connection with imaging. It is based on capturing two images at the moment of imaging. The first image is focussed on the primary image object and before the second image is captured the focussing is altered to either the close or distant setting. After taking the shots, the first and second images are synthesized with each ether. As a result of the synthesizing, a final image is obtained, in which the object is imaged sharply while the background is blurred.
Some other prior arts applying two or several images describe US 2002/0140823 A1, US 2002/0060739 A1, US 2003/0071905 A1 and US 2002/0191100 A1.